Methods of improving the performance of existing and future steppers have a large impact on the economics of microchip and flat panel display production. The ability to improve projection imaging systems through minimally intrusive retrofitting has been done using the techniques of McArthur et al. U.S. Patent entitled Plate Correction of Imaging Systems U.S. Pat. No. 5,392,119 issued Feb. 21, 1995; McArthur et al U.S. patent application entitled Improved Plate Correction Technique for Imaging Systems, Ser. No. 08/592,703 filed Jan. 26, 962; McArthur et al U.S. Patent Application entitled Single Plate Corrector for Stepper Lens Train filed concurrently herewith; and MacDonald et al U.S. Patent Application entitled Imaging and Illumination System with Aspherization and Aberration Correction by Phase Steps now U.S. Pat. No. 5,136,413 issued Aug. 4, 1992.
A key part of retrofitting projection imaging systems is quickly and reproducibly monitoring their state of optical correction. In the above references, distortion and field curvature data from exposed images are interferred and used to design figured optical surfaces that are placed between the top lens and the reticle plane. Distortion and field curvature correspond the lowest order aberrations of an imaging system, namely field dependent tilt and focus. Methods for their in-situ measurement are described in M. Dusa, D. Nicolau, In-house Characterization Technique for Steppers published in Optical/Laser Microlithography II (1989) SPIE Vol. 1088, p. 354. 1989; and D. Flagello, B. Geh entitled Lithographic Lens Testing: Analysis of Measured Aerial Images, Interferometric Data and Photoresist Measurements, SPIE Vol. 2726, p. 788 of Jun. 19, 1990.
In order to ascertain the degree of correction and method of correction for higher order aberrations, more data is necessary than distortion and field curvature.
The application of a conventional interferometer to a projection imaging system can provide high quality wavefront data(see also W. Freitag, W. Grossmann, U. Grunewald entitled Aberration Analysis in Aerial Images formed by Lithographic Lenses, Applied Optics, Vol. 31, No. 13, p. 2284, May 1, 1992).
However, conventional interferometers require removing or significantly altering or disturbing the lens column. The act of removing the lens column could well introduce uncertainties into the measurement and require significant downtime from productive operation. Therefore, in the prior art we find in-situ techniques for determining, distortion, field curvature, best focus, astigmatism, and the aerial image (M. Dusa, D. Nicolau, In-house Characterization Technique for Steppers, Optical/Laser Microlithography II (1989) SPIE Vol. 1088, p. 354 1989; L. Zych, G. Spadini, T. Hasan, B. Arden entitled Electrical Methods for Precision Stepper Column Optimization, Optical Microlithography V (1986), SPIE Vol. 663, p. 98, 1986; T. Brunner, S. Stuber entitled Characterization and Setup Techniques for a 5.times. Stepper Optical/Laser Microlithography V (1986), SPIE Vol. 663, p. 106 1986; J. Kirk entitled Astigmatism and Field Curvature from Pin-Bars, Optical/Laser Microlithography IV, SPIE Vol. 1463, p. 282, Mar. 6, 1991, A. Pfau, R. Hsu, W. Oldham entitled A Two-Dimensional High-Resolution Stepper Image Monitor, Optical/Laser Microlithography V, SPIE Vol. 1674, p. 182, Mar. 11, 1992; E. Raab, C. Pierrat, C. Fields, R. Kostelak, W. Oldham, S. Vaidya entitled Analyzing the Deep-UV Lens Aberrations Using Aerial Image and Latent Image Metrologies, Optical/Laser Microlithography VII, SPIE Vol. 2197, p 550, Mar. 2, 1994; C. Fields, W. Partlo, W. Oldham entitled Aerial Image Measurements on a Commercial Stepper, Optical/Laser Microlithography VII, SPIE Vol. 2197, p. 585, Mar. 2, 1994; C. Huang entitled In-situ Optimization of an i-line Optical Projection Lens, Optical/Laser Microlithography VIII, SPIE Vol. 2440, p. 735, Feb. 22, 1995).
The greatest amount of information is provided by in-situ aerial image measurements. However the light level is generally low leading to long exposure times or poor signal to noise ratios. The reconstruction of the aberrated wavefront is ambiguous unless several out of focus exposures are done (see D. Redding, P. Dumont, J. Yu, Hubble Space Telescope Prescription Retrieval, Applied Optics, Vol. 32, No. 10, p. 1728, Apr. 1, 1993; J. Fienup, J. Marron, T. Schulz, J. Seldin, Hubble Space Telescope Characterized by Using Phase-Retrieval Algorithms, Applied Optics, Vol. 32, No. 10, p. 1747, Apr. 1, 1993; J. Fineup entitled Phase-Retrieval Algorithms for a Complicated Optical System, Applied Optics, Vol. 32, No. 10, p. 1737, Apr. 1, 1993).
The ideal solution would minimize the intrusion, preserve the lens column and stepper environment, and be quick to perform. It could be utilized to rapidly characterize multiple steppers, as well as perform temporal, barometric, thermal and other environmental characterizations. The device should be a stand-alone, portable unit, that would determine wavefront as a function of imaging field position for all conceivable aberrations of the stepper.
The data generated using such an in-situ wavefront interferometer can be used to help determine opportunities for lens correction. In addition, the output could be utilized in commercially available software programs such as PROLITH and used for photolithography process modeling.